Photoconductive elements comprise a conducting support bearing a layer of a photoconductive material which is insulating in the dark but which becomes conductive upon exposure to radiation. A common technique for forming images with such elements is to uniformly electrostatically charge the surface of the element and then imagewise expose it to radiation. In areas where the photoconductive layer is irradiated, mobile charge carriers are generated which migrate to the surface of the element and there dissipate the surface charge. This leaves behind a charge pattern in nonirradiated areas, referred to as a latent electrostatic image. This latent electrostatic image can then be developed, either on the surface on which it is formed, or on another surface to which it has been transferred, by application of a liquid or dry developer composition which contains electroscopic marking particles. These particles are selectively attracted to and deposit in the charged areas or are repelled by the charged areas and selectively deposited in the uncharged areas. The pattern of marking particles can be fixed to the surface on which they are deposited or they can be transferred to another surface and fixed there.
Photoconductive elements can comprise a single active layer, containing the photoconductive material, or they can comprise multiple active layers. Elements with multiple active layers (sometimes referred to as multi-active elements) have at least one charge-generating layer and at least one charge-transport layer. The charge-generating layer responds to radiation by generating mobile charge carriers and the charge-transport layer facilitates migration of the charge carriers to the surface of the element, where they dissipate the uniform electrostatic charge in light-struck areas and form the latent electrostatic image.
The photoreceptor properties that determine the radiation necessary to form the latent image are the quantum efficiency, the thickness, the dielectric constant, and the existence of trapping. In the simplest case, where trapping can be neglected, the exposure can be expressed as: ##EQU1## where E is the exposure in ergs/cm.sup.2, .epsilon. the relative dielectric constant, L the thickness in cm, e the electronic charge in esu, .lambda. the wavelength in nm, .phi. the quantum efficiency, k a constant equal to 5.2.times.10.sup.-13, and .DELTA.V the voltage difference between the image and background area, V.sub.i -V.sub.b. The quantum efficiency, which cannot exceed unity, represents the fraction of incident photons that are absorbed and result in free electron-hole pairs.
For electrophotographic processes known heretofore, .DELTA.V is typically 400-500 V. Assuming typical values of .epsilon.=3.0, .lambda.=500 nm, and L=10.sup.-3 cm, the above equation predicts an exposure energy of 11.8 to 14.7 ergs/cm.sup.2. This assumes that there is no trapping and is based on the absorbed radiation. In practice, the radiation is not completely absorbed, and the exposure is correspondingly larger. Thus, most photoreceptors require exposures in the range of 20-100 ergs/cm.sup.2 to form an electrostatic image. These are equivalent to ASA ratings between 0.1 and 0.02. In contrast, the exposure required to form a latent image in conventional silver halide photography is in the range of 10.sup.-2 to 10.sup.-1 ergs/cm.sup.2, or less, and, accordingly, the radiation sensitivity of electrophotography is less than that of conventional silver halide photography by a factor of at least 10.sup.3.
While increases in electrophotographic sensitivity can be realized by increases in thickness or quantum efficiency, these effects are limited. Increases in photoreceptor thickness tend to result in trapping, which gives rise to a sharp decrease in sensitivity. Since the quantum efficiency cannot exceed unity, increases in efficiency are limited. For the example discussed in the preceeding paragraph, the maximum increase in sensitivity would be a factor of about 5. In practice, absorption and reflection losses, photogeneration efficiencies of less than unity, etc., would limit the increase to probably no more than a factor of about 3. Consequently, if the sensitivity is to be significantly increased, the magnitude of the voltage difference between the image and background areas must be reduced. Moreover, if the sensitivity is to be increased without a concurrent increase in electrostatic noise, the magnitude of V.sub.b must also be reduced, since a reduction in .DELTA.V without a corresponding reduction in V.sub.b results in a very low signal to noise (S/N) ratio.
A reduction in both .DELTA.V and V.sub.b requires that the photoreceptor be initially charged to very low voltages, e.g., V.sub.o =10 volts. However, with photoconductive elements of both the single-active-layer and multiple-active layer types, the quantum efficiency typically decreases sharply with decreasing voltage. [See D. M. Pai and R. C. Enck, Phys. Rev. 11, 5163, (1975); P. J. Melz, J. Chem. Phys 57, 1694, (1972); and P. M. Borsenberger and D. C. Hoesterey, J. Appl. Phys 51, 4248 (1980)]. As a result, electrophotographic processes typically employ a high initial voltage, such as 500 volts, and electrostatic latent image formation typically requires exposures of the order of 20 to 100 ergs/cm.sup.2.
It is toward the objective of providing a high speed electrophotographic process which exhibits minimal electrical noise, and, in particular, a low field process employing a very low initial voltage, such as a voltage of 10 volts, that the present invention is directed.